The project is designed to characterize the dynamics of small rodent populations and investigate some of the mechanisms that cause them. Random mortality and emigration rather than longevity are the major forces for decrease in natural populations. In door and outdoor enclosures with "migration" areas will be used to determine when migration occurs and who the migrants are from populations with known social structures. Factors affecting natality such as intrauterine mortality, infant mortality and adult reproductive shut off are also being examined. Preliminary results with computer analysis as well as data from natural and confined populations suggest that age at maturity (delayed maturation) is a major regulating factor in nearly all populations in the laboratory or in the wild. We have also characterized the adult reproductive shut off in greater detail. Pituitary ACTH operating through the opioid receptor sites of the CNS inhibit gonadotropins, particularly LH. We are now trying to establish that crowding and density operate through these ACTH pathways and can therefore be blocked by opioid antagonists. These data are being assembled into two computer simulations--one at the populations level and a second at the mechanism level.